The current construction of micromechanical piezoresistive pressure sensors for use in the automotive field provides for four piezoresistive resistor strips, which in each case are located at the center of the edge of a square diaphragm and are interconnected to form a Wheatstone bridge.
From U.S. Pat. Nos. 5,485,753 and 5,714,690, rectangular diaphragms are known, on which the piezoresistive resistor strips run parallel to the long edge. From K. H.-L. Chau et al., “HIGH-STRESS AND OVERRANGE BEHAVIOUR OF SEALED-CAVITY POLYSILICON PRESSURE SENSORS”, IEEE Solidstate Sensor and Actuator Workshop, Jun. 4, 1990, pages 181 through 183, rectangular diaphragms having an aspect ratio greater than 3:1 are known.
FIG. 4 shows a micromechanical piezoresistive pressure sensor device known from U.S. Pat. No. 5,714,690.
In FIG. 4, reference character 30 designates a silicon sensor substrate having a diaphragm region 36. Diaphragm region 36, for example, is a region of substrate 30 that has been thinned from the back using an anisotropic etching method.
Diaphragm region 36 has a rectangular shape with a high aspect ratio, which means that the long sides of the diaphragm edge 39 are significantly longer than the narrow sides. Piezoresistive resistor strips 35a, 35b run in the central region 91 of diaphragm region 36 parallel to the latter's long sides, that is, strip 35a near the longitudinal edge and strip 35b at the longitudinal center line 4a. Thus strip 35a is located in a region of high tensile stress and strip 35b in the region of the highest compressive stress. At their ends, strips 35a, 35b are bent and lead into bonding surfaces 31a, 32a or 31b, 32b, which are provided on the surface of substrate 30.
In the case of a rectangular diaphragm region 36, the surface requirement for achieving a specified sensitivity is greater than that in the case of a square diaphragm. In the case of a rectangular diaphragm, however, the resistance of the strip, which runs parallel to the long edge at the center of the diaphragm, is in quantitative terms approx. 50% lower. The center resistance displays over pressure a higher nonlinearity than the edge resistance.
The square diaphragm has the advantage that, although the piezoresistive resistor strip pairs differ in terms of their sign, they are almost identical in quantitative terms and do not differ in their nonlinear behavior.
The mentioned problem is likely responsible for the fact that rectangular diaphragms so far are not finding a significant use in commercial micromechanical piezoresistive pressure sensor devices.
In U.S. Pat. No. 5,714,690, moreover, piezoresistive resistor strips 35a, 35b run perpendicularly across the longitudinal edge of diaphragm region 36. This immediately reduces the sensitivity since the direction of the current is here turned by 90° with respect to the region relevant for the measurement and is thus running parallel to the mechanical stress.